Your projection choice means more than just map display
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Transcript of Your projection choice means more than just map display
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How Coordinate System and Projection choice affect ArcGIS Geoprocessing Results
WLIA Conference, 2011
Your projection choice means more than just map display
or
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2 Case Studies
1. Large Scale map – parcel selection by buffer
2. Small Scale map – Geodesic vs Euclidian buffers
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Parcel Selection by Buffer
Need list of parcels 1,000 feet from an address
Using ArcMap for the task Using World Street Map ArcGIS Map
Service as base map Parcel data from Milwaukee County
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Add World Street Map ArcGIS Map service
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Geocode focus address – 3131 S 29th St, Milwaukee
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Add address point to map
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Convert graphic to a shapefile
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Create 1,000 ft bufferfrom address point -default settings
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Create 1,000 ft buffer from address point - local coordinate system
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Comparison of different buffer outputs
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Select parcels that intersect 1st buffer (Web Mercator) – 126 parcels
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Select parcels that intersect 2nd buffer (SPCS WI South) – 239 parcels
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Geodesic vs Euclidian distance
Geodesic Distance – distance measured along Earth’s surfacea.k.a. Great circle distance
Euclidian Distance – straight line distance between 2 points on a 2D planeMeasured on the Cartesian coordinate
grid of the map projection
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Geodesic vs Euclidian distance
ArcGIS can create buffers using either distance
The difference more noticeable at smaller scales and at areas of the map with more distortion
Example - World mapEuclidian distance buffers created
using Equidistant Cylindrical projection
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